When I do my homework, I often write extra techniques or steps on the sides that aren't found in my notes. Because I scan my homework, I don't have to worry about putting it in my notes or losing my thought process after turning my homework in.
Another reason for keeping a scanned copy of your homework is this: in case your professors don't return the homework in time for an exam (or they don't return it at all), you will at least still have a reference file you can come back to for studying.
It is also a good idea to back up your homework via a scan in case you lose or damage the physical copy. Plus, in the long run, keeping digital copies of your homework is better than keeping stacks of paper!